'I'm not a charade - I'm a keep-it-real girl'

Pop diva Anastacia has a confession to make: she has been lying about her age throughout her career. As she prepares to turn 40 (not 34), she tells Caroline Sullivan about sexism, the shock of moving to Britain - and why Botox is cool

It's a Monday afternoon and in a dimly lit rehearsal studio in London, three young women are singing as if their lives depend on it. And in a way they do. They're auditioning to be backing singers for Anastacia and, if they get the job, her superstar name and the 20m album sales associated with it will kick their careers into another league.

Anastacia, a compact blonde forcefield, is sitting across the room, listening and fiddling with a pen. When the girls finish, she makes a note and the three hopefuls are ushered out, to be replaced by another trio, then another. As the last group leave, Anastacia shakes her head incredulously. "This isn't even close to the calibre I can work with," she tells an assistant. "Not even remotely. They're flat, they're sharp." Perhaps, she says, a few might be suitable for an upcoming TV appearance, where only miming will be necessary. "That might work. They'd bring a young energy."

The subject of young energy has particular significance for Anastacia these days. Having recently undergone a professional rebirth (new label, new management and, next month, a new album called Heavy Rotation), she has decided to complete the clearing-out process - by coming clean about her age. Since achieving fame in 2000 with Not That Kind, her debut album filled with assertive dance tracks and ballads sent soaring by a voice that scarcely seemed to belong to a white woman, Anastacia has maintained that she was born in 1974. In truth, it was 1968. So this Wednesday she will turn 40. It may only be a number, but for a singer who's made her mark by exuding sexy youthfulness (her visual trademarks are tousled hair and a figure that has plenty of what she calls "a booty and a rack"), 40 is a big number.

Her honesty is not without risks. Pop is one of the few industries in which ageism is openly practiced. Note the widespread, often snide, coverage when Kylie, Madonna, Michael Jackson and Mick Jagger celebrated milestone birthdays this year. Kylie is grudgingly said to be "still" attractive at 40, as if she has entered a zone where nothing awaits but deterioration. Elton John, 61, got a taste from Lily Allen when the two co-hosted the GQ Awards recently. "I'm 40 years younger than you and have my whole life ahead of me," she told him. "I could still snort you under the table," he replied.

In this context, Anastacia's decision to tell the truth is certainly brave. "My shoulders feel like a weight has been lifted off them," she says. "I'm not a charade, I'm a keep-it-real girl, but you'd be dancing around a question, trying to remember when you were supposed to have been at school."

It was initially MTV's decision to knock six years off her age. In the late 1990s, after a decade on the periphery of Manhattan's pop scene (initially a dancer, she appeared in videos for acts like Salt-n-Pepa and sang on the side), she came second in one of the channel's talent competitions. She was nearly 30, but MTV decided it would play better if viewers were told she was 24. Then she was offered a deal with Epic: "The label said, 'We'll stick with 24. You look it and it'll make you more appealing to your audience.' I didn't know my success was going to be so big and that I would become 'the subtractor', always subtracting six years. In the last few years I started saying, 'I'm in my 30s', which I thought was a good cop-out."

Outing herself has caused quite a shift in her thinking: "The female in me has come out. I feel lighter now - feline and feminine and 40. We're women in a society built by men, so we stay young and attractive for them. We don't do it for us." The ironic thing is that she has such a peachy-fresh look, she could easily have continued the deception. This she attributes to two things: "not being a partyer" and, surprisingly, Botox. "I believe in it," she says, vigorously. "I think it's great, it's cool. When I originally had it, I couldn't have been any smoother. I was, like, wow! I don't top up all the time, but once I start to look a bit tired, I have a dose. I'm doing teeny steps, so the frown lines aren't so intense, and if the creases become too crazy I'll have filler. It can work wonders."

We discuss Madonna's efforts to fight the passage of time and the sinewy body that's being shown off on her current tour. "God bless her," Anastacia says vehemently. "She loves to work out ferociously. She looks phenomenal from head to toe, but her body fat has changed. My heart goes out to her. She wants to look decent in this business, so she has to work out harder than most people her age."

Anastacia's forthrightness is striking. Most women in the belting-pop-diva category are experts at not divulging any "work" they've had done. But Anastacia is an oddball of a diva: she listens closely to questions, is refreshingly unguarded in her responses and is highly tactile, even with strangers. When we're introduced, she says, "Hi, babe!" and gives me a big hug. Such openness may come of being nearly unknown in America (she estimates that only 500,000 of her 20m sales have been in her homeland) where successful artists soon learn to be as bland as possible with the press. "Typical Americans don't know who I am," she says. "I'll meet them and they'll say, 'What do you do?' 'I'm a singer.' 'Oh, where do you sing?' 'I sing overseas.' 'What, on cruise ships?'" She laughs earthily.

There are various reasons it hasn't happened for her at home, not least the fact that J-Lo was the priority for Sony, her former label. She doesn't seem too disheartened: "If I was big in America, I wouldn't have had the career I've had, and wouldn't have needed a bodyguard, and wouldn't have ended up meeting Wayne." East London-born Wayne was Anastacia's head of security for three years before being promoted to husband in 2007. He's the reason she has relocated from LA to a west London flat, which has been a bit of a culture shock: "The service here! I have to sit in the internet cafe, because we can't get our internet connected." LA remains the couple's first home, she says, but having a place here makes sense since her main audience is in Europe, and it lets Wayne see more of his two sons, who live in Kent.

The pair seem blissful. At the auditions, he was a beaming, protective presence, while she says she feels "honoured" to have changed her surname, Newkirk, to his, Newton: "I love being his wife. I take marriage as seriously as it's supposed to be taken."

I'm curious to know how Wayne takes it when Anastacia overrides him. He said positive things about one of the auditioning singers and Anastacia fiercely replied: "No, honey, she was off the fuckin' beat - she was horrible dotcom." She says now: "When I'm working, it appears that I wear the pants in the relationship, because he's in security and his job is to blend in." More likely, Anastacia is just skilled at making career decisions, and doesn't need his input. Although several songs on Heavy Rotation - a solid album produced by a cast of heavy-hitters including Ne-Yo and Rodney Jerkins, employed partly in the hope of breaking her in America - are about their marriage, she seems strongly independent, and quite capable of looking after herself.

A string of illnesses - breast cancer, the bowel disorder Crohn's disease, and now a heart ailment that causes palpitations - doubtless contributed to her hardiness. "Healthwise," she says, "I'm doing very well. I'm heading into my fifth year of being clean of cancer. I only had one biopsy scare: they detected something, and they went in but it was fine. It was a real butt-kicker, because it happened at Christmas and I was, like, 'Noooo!'" The Crohn's she has had since the age of 12: "Early in my career, a producer said I should be thinner, so I ate nothing but fruit and vegetables and lost 20 pounds in about three weeks. I didn't realise that was something you shouldn't do with Crohn's. I ended up in the hospital for three weeks on a steroid IV drip. It didn't help my self-image."

Anastacia's songs are shot through with triumph-over-adversity messages. Although that's a standard theme in US chart-pop, she has more reason than most to deliver onward-and-upward lyrics. One track on the new album seems to sum her up perfectly: "You can tear me apart," she belts out, "you can rip me to pieces, you can try to break me down, but I'll never be beaten".

The divas who defy the years

· Madonna is treating middle age as just another glass ceiling to smash through. Thanks to a regime of furious exercise, she's changed our idea of what's physically achievable for a 50-year-old woman. The result is a body seemingly hewn from granite and a face that looks, according to the tabloids, either "suspiciously smooth" or "gaunt".

· Cher got her big break in the 1960s as Sonny Bono's teenage wife, but was aided greatly in her solo career by her part-Native American looks and streamlined body. She never hesitated to distract attention from her foghorn voice with revealing clothes. Now 61, and evidently no stranger to cosmetic surgery, she recently told Oprah Winfrey that getting older "sucks".

· Madonna's great rival in the early 1980s, Cyndi Lauper, somehow ended up the also-ran. Despite possessing the superior voice and quirkier fashion sense, Lauper fell short perhaps because she never traded on her looks. But she's blossomed in middle age, looking better than she did in 1983.

· The young Debbie Harry was arguably the most beautiful of all pop singers. Her kittenish face still appears on T-shirts and posters. But although her magnificent bone structure makes her a striking 63-year-old, she continues to wear the fishnets and punk clothes of the 1970s and cuts a slightly sad figure.

· Youth can become a curse if your whole career pivots on it. Kylie may be growing older with grace (and possibly a little help), but is so associated with pixieish cuteness that turning 40 in May can't have been easy.